This study has shown that a mouse antiidiotypic mAb bearing a mirror image of HMW-MAA can break tolerance to a well-defined self HMW-MAA and can induce humoral anti HMW-MAA immunity in patients with melanoma. Furthermore, the induction of an immune response to a well-defined melanoma-associated antigen has been shown to be associated with a favorable clinical response. If this association reflects a cause-effect relationship between the two parameters, the anti HMW-MAA immunity induced by mouse antiidiotypic mAb MK2-23 may effect immune destruction of melanoma cells and may interfere with the metastatic potential of those cells because of the suggested role of HMW-MAA in this phenomenon.